Sleep and wake behaviors are generated by an interaction of one's circadian rhythm, as well as social and environmental factors. A circadian rhythm is a self-sustaining biological activity oscillating with a periodicity near 24 hours. Circadian rhythms tend to move into phase synchrony with environmental rhythms, mainly the day/night cycle.
Light therapy has been shown to alter one's circadian rhythm. Therapy can be used to shift the biologic clock phase to facilitate sleep at a desired time of day/night. The biologic clock is, however, not equally amenable to phase shifts throughout its circadian period. Successful light therapy, requires the identification of circadian windows of opportunity for intervention.
Light therapy can yield phase advancement (positive shift), delay (negative shift), or be entirely phase neutral depending on the biologic clock time at which the light is administered. The biologic valence of light therapy is determined by 2 inherent features—wavelength and intensity. Visible light has an approximate wavelength spectrum of 380 (violet) to 760 (red) nm. It has been demonstrated that short wavelength blue light (˜460 nm) possesses greater phase shifting properties than the rest of the visible light spectrum. The unit of intensity for visible light is lux. For example, the intensity of sunlight at midday measures over 100,000 lux.